In making bridge columns supporting bridges, elevated roads, etc., various materials including concrete are conceivable. Bridge columns made of steel are an example.
Generally, steel bridge columns are constructed by welding ends of thick plates of steel to form corner joints, thereby constructing a structure with a closed rectangular (square, for example) cross-section.
In welding the ends of thick steel plates to form corner joints, arc welding (SAW, SMAW, GMAW, etc.) is frequently used for reasons including ease of welding.
In forming a corner joint by arc welding, it is common to form a bevel at an end of one of the two thick steel plates to be joined, and form an accumulation of weld metal in a groove defined by the bevel. When such bevel is formed, arc welding cannot be applied to a root portion, so that a non-welded portion (unmelted portion) remains, so that the corner joint is formed as a partly-welded joint. Also when the corner joint is formed as such partly-welded joint, the corner joint needs to have sufficient rigidity and strength. Research shows that limiting the size of the root portion measured along the plate thickness to, for example ¼th the thickness of the thick steel plate or below, thereby ensuring a sufficiently large bevel leads to a desired result (see “Estimation for corner joint strength of structural member that under the ultimate state” in “Preprints of the National Meeting of Japan Welding Society” vol. 72, pp. 250-251 (published on Mar.24, 2003)).
Recently, laser beam welding (LBW), which welds metal members by heating and melting them using a laser beam, has been developed and is being put to practical use. For example, there is publicly known a welding method in which arc welding and laser beam welding are applied to metal members with a single-V groove defined between them (see Japanese Patent Application KOKAI Publication Hei 6-114587).
As disclosed in “Preprints of the National Meeting of Japan Welding Society” vol. 72 mentioned above, in arc welding, limiting the size of the root portion measured along the plate thickness to ¼th the plate thickness or below, thereby ensuring a sufficiently large bevel, particularly with the intention of forming a corner joint as a completely-welded joint, leads to sufficient rigidity and strength of the corner joint, on the one hand, but on the other hand, leads to an increase in welding work, high costs, and an increase in weld metal corresponding to an increase in size of a groove defined by the bevel. The increased amount of weld metal leads to an increase in thermal shrinkage and therefore an increase in residual stress, and invites an increase in thermal deformation due to excessive heat input.
Further, the corner joint formed as a partly-welded joint including an unmelted portion cannot have sufficient quake resistance, even though it can support a normal load.
In the welding method disclosed in Japanese Patent Application KOKAI Publication Hei 6-114587 mentioned above, laser beam welding and arc welding are applied to plates to form a butt joint. Specifically, in the welding method disclosed in this publication, laser beam welding is applied to a root portion of a single-V groove, from the root-portion side (back side) of the plates, and arc welding is applied to bevels thereof, from the bevel side (front side) of the plates.
The corner joint of the steel bridge column distinct from a simple butt joint, however, does not allow direct application of the welding method disclosed in the above-mentioned publication.
Further, the technique disclosed in the above publication aims chiefly to improve shape accuracy, and does not solve the problems as mentioned above.